The present invention generally relates to a voltage controlled oscillator circuit and, more particularly, to an adjustable voltage controlled oscillator circuit, such as is employed in a mass air flow meter, for converting a voltage input to a frequency output.
Referring to FIG. 1, an air flow meter 10 is shown positioned at the output of an air cleaner 12 and at the intake side of a throttle 14 for measuring the amount of air passing into an internal combustion engine 16, such as for use in an automotive vehicle. The air flow meter generates a voltage signal proportional to the mass air flow. The internal combustion engine 16 is generally controlled by an engine control unit (ECU) 18. The air flow meter 10 includes a voltage controlled oscillator (VCO) circuit 20 for converting the voltage generated internally by the air flow meter 12 to a frequency signal f for processing by the ECU 18. Accordingly, the air flow meter 10, with VCO circuit 20, allows for measurement of the amount of air passing into the internal combustion engine 16 to allow the ECU 18 to control the amount of fuel being injected so as to regulate the air-to-fuel ratio in the combustion chamber.
A conventional voltage controlled oscillator circuit 20 for use in the air flow meter 10 is illustrated in FIG. 2. The VCO circuit 20 includes an operational amplifier 28 having a negative terminal (xe2x88x92) coupled to a voltage input VB via a series of multiple external resistors, identified herein by resistor 24. The positive terminal (+) of the operational amplifier 28 is coupled to reference voltage input VREF. A capacitor 30 is coupled between the output 26 and the negative terminal of amplifier 28. The operational amplifier 28, resistor 24, and capacitor 30 form an integrator that generates a ramp voltage, identified as RAMP. In addition, the conventional VCO circuit 20 includes a comparator 32 that compares the output 26 of amplifier 28 to +1 volt, and provides an output to an inverter 34. The inverter output is connected to the enable input of a oneshot monostable multivibrator circuit 36 which, in turn, generates a frequency output signal f at output 44. The oneshot circuit 36 is coupled to capacitor 40 and resistor 38, both of which are coupled to ground. The negative terminal of the amplifier 28 is further coupled to a current source 46 via switch 42.
The current through the resistor 24 is referred to herein as IIN. When the ramp voltage is below one volt, the comparator output is low. When the ramp voltage is above one volt, the comparator output is high. The comparator output is applied through the inverter 34 to the oneshot circuit 30. The oneshot circuit 36 generates a positive pulse when the input thereto asserts a positive signal. When the output frequency signal at output 44 is high, the current IS generated by current source 46 is applied to the negative terminal of the amplifier 28, thereby causing the output 26 of the amplifier 28 to ramp with a positive slope. The negative slope of ramp signal increases with increased input voltage VB to change the frequency output.
One example of the ramp voltage (RAMP) at amplifier output 26 and output frequency signal 44 generated by the conventional VCO circuit 20 is illustrated in FIG. 3. The ramp voltage decreases in amplitude on negative slope 52 until output signal 44 is pulsed high. This occurs when the ramp voltage crosses below one volt at which time the current source IS is applied to the negative terminal of the amplifier 28, and ramp voltage begins to ramp up on positive slope 50. The positive slope 50 of the ramp voltage is maintained until the output signal 44 asserts low. At that point, the ramp voltage slopes negative again and one period T of oscillation is complete.
The oscillation period (T=1/f) generally is inversely proportional to the input current IIN, and is proportional to the capacitor 40 in the oneshot circuit 36. The VCO circuit 20 is generally stable, with two elements primarily factoring into the frequency equation: the external input resistor 24; and the external oneshot capacitor 40. However, the conventional VCO circuit 20 suffers from a number of drawbacks that limit overall accuracy and linearity. First, the operational amplifier 28 has a finite bandwidth and slew rate. There exists a rapid change in the ramp voltage when the oneshot pulse is asserted and de-asserted. This generally causes errors in the ramp voltage and is a source of overshoot 48 and rounding errors. These errors generally result in non-linearity in the voltage-to-frequency relationship. Secondly, the rapid changes in the amplifier output generally require significant current from a regulated voltage supply. This increases the complexity of the regulator design and increases the regulator output noise. Third, the oscillator frequency typically can only be modified by adjusting the value of the external resistor 24 or the value of reference voltage VREF which is typically generated by an external voltage source and a resistor divider network. The standard approach for adjusting an external resistor has been to perform laser trimming which is generally expensive and is implemented when the device is not under flow. This requires an independent measurement and trim process in the manufacturing flow which are costly and limit accuracy of the overall response. Finally, the VCO circuit output is not at a fixed duty cycle. This requires the response time of the engine control unit monitoring the frequency to be as fast as the narrowest pulse possible out of the VCO circuit, rather than half of the minimum. This can be a problem, particularly at high input voltages VB.
Accordingly, it is therefore desirable to provide for a voltage controlled oscillator circuit with reduced error signals. It is also desirable to provide for a voltage controlled oscillator circuit that can be easily adjusted without requiring laser trimming. It is further desired to provide for a voltage controlled oscillator circuit that reduces the amount of current supply required from the regulated supply. Yet, it is further desirable to provide for a voltage controlled oscillator circuit having a fifty percent duty cycle.
The present invention provides for an adjustable voltage controlled oscillator circuit for converting a voltage to a frequency. The voltage controlled oscillator comprises an input for receiving a voltage input signal, an integrator coupled to the input for receiving the voltage input signal and generating a ramp signal. According to one aspect of the present invention, the circuit also includes an adjustable current supply coupled to the integrator for supplying an adjustable amount of current. A comparator compares the ramp signal with a predetermined voltage. The circuit further includes an output for generating a frequency output signal as a function of the comparison, wherein the oscillator circuit is calibratible by adjusting current generated by the adjustable current supply. According to another aspect of the present invention, a first current source supplies current to the input of the integrator, and a second current source supplies current to the output of the integrator.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.